Field of the Invention
The present invention relates to a flash band determination device, a method of controlling the same, a storage medium, and an image pickup apparatus, and more particularly to a flash band determination device for detecting a flash band, that determines whether or not there is a flash band caused by an external flash of light in an image obtained through image pickup.
Description of the Related Art
In recent years, as an image pickup device used in an image pickup apparatus, such as a video camera, a CMOS image sensor (hereinafter referred to as the CMOS sensor) is used. In general, compared with a CCD image sensor (hereinafter referred to as the CCD sensor), the CMOS sensor is advantageous not only in that it is easy to manufacture, but also in that it consumes less power. Further, there is also proposed a CMOS sensor increased in frame speed of reading one frame by increasing the number of read-out channels.
The CMOS sensor uses a so-called rolling shutter method in which horizontal lines are sequentially exposed, and video signals (image signals) are sequentially read out on a line-by-line basis to thereby generate one frame. In the CMOS sensor using the rolling shutter method, the exposure timing for each line and the read-out time for each line are out of synchronization.
For this reason, if there is an external flash of light emitted in a very short emission time period, such as a flash or a strobe light, there is generated, depending on a location on the screen, an image in which one frame is divided into an upper belt-like area and a lower belt-like area different in brightness, over two frames. Such a step of brightness between the belt-like areas caused by the external flash of light is referred to as the flash band.
On the other hand, in a case where the CCD sensor is used, the above-mentioned flash band is not generated, so that even when an external flash of light is used, it is possible to obtain an image which is uniformly bright throughout one frame and is free from a sense of strangeness. For such a reason as described above, an image pickup apparatus using the CMOS sensor is equipped with a correction function for correcting a flash band.
For example, there has been proposed an image pickup apparatus configured to detect a flash band so as to correct the same (see e.g. Japanese Laid-Open Patent Publication (Kokai) No. 2009-253367). This image pickup apparatus detects whether or not there is a high-luminance time period corresponding to one frame, and if the detected high-luminance time period corresponds to one frame, it is determined that a flash band has occurred.
Incidentally, to detect a flash band caused in each frame over two frames by an external flash of light, it is required to detect a change in line average luminance in a horizontal direction in an image, and determine whether or not the change in luminance continues over two frames. However, if the light amount of the external flash of light is not large enough, when there is a change in the aperture of a lens, a gain of the image pickup device, or the like, the level of the flash band is changed between the two frames. As a result, the level of the flash band is sometimes lower than a threshold level for determining a high luminance, which reduces the accuracy of flash band detection.